1. Field of the Invention
The present invention relates to methods and apparatus for measuring angular rates. More particularly, this invention pertains to a new method and apparatus for measuring rotation based upon the Sagnac phase shift of beams upon counterrotation within a fiber optic coil.
2. Description of the Prior Art
In a fiber interferometer gyroscope operating in accordance with the Sagnac effect, the phase difference, over one rotation, of two component counterrotating light fluxes, may be compensated by modulating or controlling the frequency difference between the component light fluxes. The phase shift occasioned by a single rotation is thus compensated by a regulated frequency shift. The possibility of such phase compensation has long been known in principle. See for example, R. F. Cahill and E. Udd, "Phase-nulling Fibre Optic Laser Gyros", Opt. Lett. Vol. 4, No. 3 (1979). Unfortunately, proper frequency shifting cannot presently be carried out in the desirable very low frequency range as conventional single-sideband modulators (Bragg cells) are only available above about 20 MHz. The large Sagnac phase difference resulting from such a large frequency shift permits only high-order compensation if one single-sideband modulator is employed. Since phase difference is proportional to the optical length of the fiber, an unacceptably large amount of temperature-dependent null drift results. One solution to this problem is disclosed in Federal Republic of Germany patent publication (Offenlegungsschrift) Ser. No. 29 34 794. Two single sideband modulators are utilized therein. As a result, the actual frequency differential between the counterrotating beams in the fiber coil remains much lower than the frequency of a single modulator. In comparison with an interferomoter utilizing a single modulator, measuring accuracy and stability can be improved at most by a factor of about 33. This result is disclosed in detail in Ser. No. 29 34 794.
It would be highly desirable to further increase measurement of angular rates by a fiber interferometer gyroscope. It would be additionally advantageous to increase accuracy while simplifying the construction of the measuring and read-out arrangement, and, further, to minimize the effects of temperature-dependent changes of the null drift, the scale factor and other parameters of the interferometer.